We apologize for the inconvenience. pravin_sloka is correct the paragraph should read: "To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding scheme known as 256-QAM. This scheme can transmit 33% more data as the 64-QAM used in the 802.11n standard."

"To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding scheme known as 256-QAM. This scheme can transmit four times as much data as the 64-QAM used in the 802.11n standard."
How can the data rate be 4x when the modulation scheme is just changed from 64-QAM to 256-QAM? 64-QAM serves 6 bits per carrier while 256-QAM does 8 bits. Thus the improvement in throughput due to 256-QAM is only 2/6 = 33% more compared to 802.11n.